LOL4R came about when LOL (It's original definition was "Laughing out loud" see:lol) became overused to the point where nobody laughs out loud when they say it.
Therefore, LOL4R (meaning "laughing out loud for real") states that you, infact, found the recipient funny/commical/hilarious and didn't use a half-arsed attempt at filling conversation.
1. George Bush does a great job.
2. LOL4R. oh dude...almost had me there.
Meaning: Laughing Out Loud For Real
you are so funny I just LOL4R